The present invention relates to a process for radioactive decontamination of metal by electrolytic polishing of the metal surface of radioactively contaminated equipment or parts used, for example, in nuclear plants or other facilities handling radioactive substances. It also relates to a process for recovering, by electrodeposition capture, radioactive metal ions in the form of solid metal, which ions dissolve in an electrolyte during the process of the electrolytic decontamination, and reproducing decontaminating electrolyte having the initial concentration.
Equipment, parts and piping used in nuclear plants are frequently contaminated by diffusion and deposition of radioactive conjugated oxides (which may be called "CRUD") and other radioactive substances as the plants are operated.
Radioactively contaminated equipment can be decontaminated by blasting the equipment with ice or dry ice, a high pressure jet of water, ultrasonic cleaning, chemical polishing or electrolytic polishing. The electrolytic polishing is the most advantageous method in respect to decontamination and prevention of recontamination, but it presents some problems in the disposal of waste electrolyte.
The major portion of the radioactive substances that contaminate metals is contained in the CRUD. The CRUD is composed of radioactive conjugated oxides which are hard to dissolve in an electrolyte. In an electrolytic polishing process, it is possible to separate the CRUD by allowing a DC current to flow between one or more cathodes and an anode formed by the contaminated metal part, the anode and the cathode being dipped in an electrolyte, so that a very thin surface layer of the metal part under the CRUD dissolves in the electrolyte. In the course of the electrolysis, metallic ions are eluted from the surface of the metal part, oxygen bubbles are generated and the electrolyte permiates into the CRUD, so that the CRUD is loosened from the metal part and dispersed in the electrolyte as suspended substances. Various radioactive substances dissolve and accumulate in the electrolyte during such electrolytic decontamination. Among them, metal oxides separated from the contaminated part and suspended in the electrolyte can be relatively easily taken out of the electrolyte in a recycling system by employing a solid-liquid separation method such as filtration of the electrolyte or separation by sedimentation.
On the other hand, radioactive substance eluted from the contaminated part and existing in the form of metallic ions in the electrolyte cannot be removed by the solid-liquid separation methods mentioned above and therefore they gradually accumulate in the electrolyte, thus increasing the radiation level of the electrolyte. If electrolytic decontamination is continued using an electrolyte in this state, workers are possibly exposed to the radiation and the service life of the electrolyte comes to an end because the electrolytic polishing efficiency is reduced as the concentration of metallic ions dissolved in the electrolyte is increased.
A dilute aqueous solution of a strong acid such as diluted surfuric acid may be used as an electrolyte for electrolytic polishing decontamination as described. This solution effects rapid polishing and it is easily disposed of after use. The surface polished using this solution is, however, rough and consequently easily contaminated again. Therefore, use of this solution is limited only to contaminated parts that are to be disposed of rather than reused. Of the high concentration acid solutions generally used for electrolytic polishing high concentration sulfuric acids yield a reduced glossy polished surface, but high concentration phosphoric acids and high concentration phosphoric acids-sulfuric acids yield a more glossy surface. Therefore, they are quite effective in preventing recontamination of equipment desired to be reused, though there has been a problem in disposal of the waste electrolyte. Specifically, various methods have been presented for isolating metallic ions accumulated in high concentration in the electrolyte during decontamination, although it has been considered difficult to isolate concentrated metallic ions dissolved in the electrolyte when a high concentration acid solution is used as the electrolyte.
One of the known isolation methods is to separate metallic ions by allowing them to deposit on a cathode capture electrode in an electrolytic cell provided with a partitioning diaphragm such as an unglazed plate or an ion exchange membrane. In this case, the reaction that generates hydrogen gas at the capture cathode electrodes takes place prior to the reaction for metal deposition on the capture electrode in the cathode chamber partitioned by the diaphragm, and therefore it is necessary to lower the hydrogen ion concentration to such an extent as to permit metal deposition. In an electrolyte of high concentration acid solution, however, acid is diffused into the cathode chamber due to large gradient of concentration between the anode and the cathode chambers partitioned by the diaphragm. As a result, it is not possible to lower the hydrogen ion concentration to such an extent as to permit metal deposition, and the diaphragm cannot have an expected effect.
Because of the above reasons, a high concentration acid solution used as a decontaminating electrolyte is conventionally solidified in plastic or cement for disposal, when the concentration of metallic ions dissolved in the electrolyte or the radiation level of the electrolyte increases to a certain value. Such disposal of the waste electrolyte presents another problem from the increasing quantity of waste which causes secondary contamination.
It is a primary object of this invention to present various methods of isolating metallic ions dissolved in an electrolyte during the process of electrolytic decontamination, in the state of the highest possible concentration, and of reproducing the electrolyte so as to minimize the volume of the secondary waste.